1. Field of the Invention
The present invention relates to a variable capacity type encoder and, more particularly, an improved variable capacity type encoder which electrically detects the rotary displacement of a rotary disc.
2. Description of the Prior Art
Various displacement measuring devices are known which use an encoder. Such a displacement measuring device includes a probe which moves on the baseplate of the device to comes into contact with an object to be measured, measures the amount of the movement of the probe and digitally indicates that value.
As such kind of encoder, a variable capacitance type encoder is known, and a smaller-sized and lighter variable capacitance type encoder has been in demand in terms of the improvement of the portability and operability.
FIGS. 8 and 9 show an example of conventional variable capacitance type encoders. This encoder, which includes a rotary disc 10 which is rotatably mounted on the baseplate (not shown) by a rotary shaft 10a which rotates in accordance with the amount of movement of a probe, and a fixed plate 12 attached to the baseplate so as to be opposed to the rotary disc 10, detects the rotary displacement of the rotary disc 10 in relation to the fixed plate 12.
For this purpose, the fixed plate 12 is provided with a plurality of electrode elements 14a which are annularly arranged on the surface thereof at regular intervals in the circumferential direction. These electrode elements 14a constitute a transmitting electrode 14. To each of the electrode elements 14a an alternating voltage of a sine wave or a rectangular wave having a predetermined phase difference, 45 in this example, is applied by a voltage supply circuit 16. In this example, alternating voltages with the phases shifted 45 degrees from each other are applied to the respective electrode elements, whereby plural groups of electrode units 100 are formed, each unit consisting of eight-phase electrode elements 14a.
Receiving electrodes 18 of the same number as the electrode units 100 are provided on the surface of the rotary disc 10, such that each receiving electrode 18 is opposed to a predetermined number of consecutive electrode elements 14a incorporated in each electrode unit 100.
In the encoder shown in FIG. 9, the receiving electrode 18 is disposed extendingly in the circumferential direction so as to be opposed to a group of four continuous electrode elements, namely, four electrode elements 14a to which the reference voltage V.sub.1, the voltages V.sub.2, V.sub.3 and V.sub.4, which are the voltages 45.degree. , 90.degree. , and 135.degree. out-of-phase, respectively, with respect to the reference voltage V.sub.1, are respectively applied.
A ground electrode 20 is interposed between every two adjacent receiving electrodes 18 on the surface of the rotary disc 10 in order to exclude the deleterious influence of the interference of the electrostatic capacities from the receiving electrodes 18 and the like.
In the encoder having the aforementioned structure, the rotation of the rotary disc 10 initiates the relative movement of the transmitting electrode 14 and the receiving electrodes 18, whereby, as is known, an electrostatic capacity signal V.sub.0 having periodic change corresponding to the rotary displacement of the rotary disc 10 is detected from the receiving electrodes 18.
In order to fetch the voltages V.sub.0 obtained by the receiving electrodes 18 on the rotary disc 10 to the baseplate side, a ring-like output electrode 22 is provided on the inside of the transmitting electrode 14 on the surface of the fixed plate 12. The receiving electrode 18 is disposed extendingly in the radial direction so as to be opposed to both the transmitting electrode 14 and the output signal 22.
This encoder having the above-described structure enables the capacity signals V.sub.0 obtained by the receiving elements 18 to be output to the baseplate side through electrostatic coupling without the need for the mechanical contact between the rotary disc 10 and the fixed plate 12, and the rotary displacement of the rotary disc 10 to be measured accurately.
The conventional variable capacitance type encoder is, however, disadvantageous in that the structure having only one transmitting electrode 12 and one output electrode 22 arranged in a ring-like form on one fixed plate 12 precludes the possibility of the fixed plate 12 being reduced in size while maintaining a high measuring accuracy and detecting resolution, and as a result it is impossible to make the encoder smaller in the widthwise direction of the fixed plate 12.
More particularly, when both the transmitting electrode 14 and the output electrode 22 are provided on one fixed plate 12 in this way, deleterious influence of the interference of electrostatic capacities is likely to be produced between both electrodes 14 and 22. To eliminate these problems, it is necessary to dispose the transmitting electrode 14 and the output electrode 22 with a large space therebetween and to interpose a ring-like ground electrode 24 between them. Therefore, it is inevitable for the conventional encoder to have the large-sized fixed plate 12 having a complicated electrode structure on the surface. This large-sized fixed plate 12 constitutes a barrier to miniaturization of the encoder in the radial direction of rotation.
To remove such influence of the interference of electrostatic capacities and enable the miniaturization of an encoder in the radial direction of rotation, application of Japanese Patent Application No. 228785/1984 was filed on Oct. 29, 1984 by the present inventor Koji Sasaki and which was published on May 23, 1986 with Japanese Patent Laid-Open No. 105,421 /86.
The variable capacitance type encoder disclosed in the above-described specification is shown in FIGS. 10 and 11. The same numerals are provided for those elements which are the same as those in the prior art shown in FIGS. 8 and 9, and explanation thereof will be omitted.
This encoder includes the rotary disc 10 which is rotatably mounted on the baseplate by the rotary shaft 10a, a first fixed plate 30 and a second fixed plate 32.
FIG. 11 shows the surface structure of each of the first fixed plate 30, the rotary disc 10 and the second fixed plate 32. In FIG. 11, the surface of the rotary disc 10 is shown in the exploded view with the surface facing the first fixed plate 30 separated from the surface facing the second fixed plate 32.
On the surfaces of the first fixed plate 30 and the rotary disc 10 are formed the transmitting electrode 14, and the receiving electrodes 18 and the ground electrodes 20, respectively.
A coupling electrode 34 is provided on the surface of the rotary disc 10 facing the second fixed plate 32 and is electrically connected to every receiving electrode 18 in order to fetch the voltage V.sub.0 obtained by the receiving electrodes 18 to the baseplate side. The coupling electrode 34 is formed on the surface of the rotary disc 10 into a ring-like shape in the circumferential direction.
The ring-like output electrode 22 is provided on the surface of the second fixed plate 32 so as to be opposed to the coupling electrode 34, whereby the output electrode 22 outputs the electrostatic capacity signal V.sub.0 induced at the receiving electrodes 18 by electrostatic coupling of the output electrode 22 and the coupling electrode 34.
By comparing the signal V.sub.0 output from the output electrode 22 with the reference voltage V.sub.1 which is set in a detecting circuit 39, the rotary displacement of the rotary disc 10 is detected on the basis of the phase difference .phi..
In this encoder, as described above, the transmitting electrode 14, the receiving electrodes 18 and the coupling electrode 34, and the output electrode 22 are provided on the surfaces of the first fixed plate 30, the rotary disc 10 and the second fixed plate 32, respectively. Accordingly, each electrode provided on the surfaces of the fixed plates 30, 32 and the rotary disc 10 is free from the interference of the electrostatic capacities from other kinds of electrodes. Thus it is possible to make the electrodes 14, 18, 34 and 22 smaller to the extent of maintaining the detecting accuracy.
Particularly, since the interference of the static capacities of the transmitting electrode 14 and the output electrode 22 is ignored in this encoder unlike that in the prior art encoder shown in FIGS. 8 and 9, it is possible to make the radius of the transmitting electrode 14 much smaller than that in the prior art to obtain the same detecting accuracy, which fact enables the encoder to be reduced in size in the radial direction of rotation.
Incidentally, since this encoder is provided with the second fixed plate 32, the thickness of the encoder in the direction of the rotary shaft 10a is increased by that thickness in comparison with the prior art encoder. However, since each space between the rotary disc 20 and these fixed plates 30 and 32 is as small as about 1/10 mm, the increase in thickness in the axial direction resulting from the attachment of the second fixed plate 32 is almost negligible.
In order to improve the detecting resolution of this type of variable capacitance type encoder, it is necessary to increase the number of electrode elements 14a which constitute the transmitting electrode 14 while maintaining the area and intervals between each electrode element 14a at an appropriate value.
Therefore, in a conventional encoder having the transmitting electrode 14 consisting of the annularly arranged electrode elements 14a, enhancement of the detecting resolution necessitates increase in the radius of the transmitting electrode 14. In other words, the conventional encoder cannot meet both the demand for miniaturization of the encoder in the radial direction of rotation and the demand for improvement of the detecting resolution.
Furthermore, in variable capacitance type encoders, it is difficult to produce the rotary disc 10 and the first and second fixed plates 30, 32 without generation of eccentricity, whirling, offset or the like, which functions as the factor in producing an error in measuring.